Using a chain til it breaks will be very costly. Chains are dirt cheap compared to chainrings and cogs.
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user313Feb 11 '11 at 17:27

2

Another thing about using a chain "til it breaks" is that it can cause you to crash when it breaks. Or even when it starts slipping before it breaks. Another good reason to maintain your chain and replace when needed...you probably don't want to find yourself lying beside some desolate road in the cold and wet all because of a slipping/broken chain.
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user313Feb 11 '11 at 21:38

Sharp spikes, as in a new chain won't fit right and will wear out very quickly. If you let the chain wear out too quickly you have to replace the front and back gears entirely. It's significantly cheaper to replace the chain before it's too worn down than to replace everything else.
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freiheit♦Feb 11 '11 at 17:18

any idea what is the general term for Rohloff Caliber? There must be some cheaper substitutes, seen many of this kind of tools in my LBS (apparently for different types of chains). Idea whether there is some do-it-all-tool to measure chain strech? Thinking...have to develop some mnemonic about stretching: when I compared my old chain to my new chain (same amount of pins), the old had stretched about 5cm. The Rohloff tool Caliber indicated chain renewing (according to my LBS). The ruler-pen-paper is more reusable but this caliber may be easier to use.
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user652Mar 11 '11 at 16:39

When the chain has "stretched" it will no longer sit properly on the teeth found on the sprockets and chainrings, symptoms of this point being reached include the chain jumping off the chainring when sufficient pressure is applied to the pedal. This usually happens when you're trying to pull away after stopping (eg at a junction). Not only is this a pain, but it can also be both dangerous and painful (my foot tended to come off the pedal and the calf/shin smacked into the teeth on the chainring when this happened to me).

The problem with leaving the chain until it gets to this state is that it also wears the sprockets at the back and the chainrings at the front. If you allow this to happen, then even buying a new chain won't do you much good as the worn teeth on the other parts of the drive system will prevent the new chain from sitting properly, and may cause the same kinds of problems as were being experienced with the old chain. If you're lucky enough not to have a problem with the chain skipping, the worn teeth will still cause the chain to wear out much faster than normal.

So, check you chain fairly regularly, if as mgb says, the chain stretched by more than 1/8 of an inch per foot, change it. Otherwise you might have to replace either the chainrings, the rear cassettes or both. A much more expensive proposition all round especially if you've got a decent chainset fitted.